Freeze-refining method



June 7, 1966 L. E. OLDS FREEZE-REFINING METHOD Original Filed Oct. 23,1962 Heat Exchange INVENTOR Leonard E. Olds United States Patent Office3,254,989 Patented June 7, 1966 3,254,989 FREEZE-REFINING METHOD LeonardE. Olds, Niagara Falls, N.Y., assignor to Inde- 4 Claims. or. 75-68)This is a division of application Serial No. 232,449, filed October 23,1962.

This invention relates in general to freeze-refining and moreparticularly to the treatment of two-component l quid systems such as,for example, molten alloys and liquid organic mixtures, whereby tworelatively pure fractions are produced through the controlled freezingof a portion of the mixture, the frozen portion comprising thehigher-melting material and the non-frozen portion comprising thelower-melting material. The invention is equally applicable to theseparation of one or more components from a multi-component system.

Heretofore, several methods of separating higherand lower-meltingfractions from systems containing two or more components have beenemployed Perhaps the most well-known of these is the process known aszone refining, wherein a rod of relatively pure metal is subjected tothe refining action of a molten Zone traversing the length thereof. Inthis process, diffusion acts to sweep lower-melting impurities to oneend of the rod, the impurities having a natural tendency to stay withinthe molten zone as it traverses the rod, migrating away from theadvancing solid face. With multiple passes of the molten zone, it ispossible to achieve extremely high purities when high-melting alloys aretreated, and the process finds ready application, for example, in thepurification of germanium and other metals intended for use insemiconductor devices. Of course, since diffusion is a time-dependentphenomenon, the effectiveness of purification will increase with adecrease in the speed of travel of the molten zone; the process is thusnecessarily a slow one. Also, equipment and handling problems limit thediameter of the rod which can be treated to about a half-inch or less;the volume of material which can be treated is, therefore, quite small.

Other freeze-refining processes have been developed which are notdependent on time-consuming diffusion and which can treat a substantialvolume of material. These may be called dynamic processes, in that theyall eliminate the need for diffusion by vigorously agitating the moltenmaterial during the course of freezing and provide some mechanical meansfor separating the solid, highermelting fraction. Typical amongthedynamic freeze-refining processes are ones in which aninternally-cooled impeller is rotated at a high rate of speed within amelt maintained at a certain temperature. Heat is extracted from theliquid in contact with the impeller, and the higher-melting fraction iscaused to freeze thereon. Baffles of some sort are generally employedwithin the containing vessel to insure turbulent motion of the liquidand prevent formation of a vortex.

A variation of the foregoing process involves the use of a large,rotating drum into which the molten material is poured and the outsideof which is continuously cooled. Heat extracted from the exterior causesfreezing of the higher-melting fraction and centrifugal force causessolidified particles to collect on the interior periphery of the drum.After a period of time, the remaining liquid is poured off and thesolidified fraction is removed either by mechanical means or byremelting.

While the foregoing dynamic processes are capable of treating relativelylarge volumes of material in reasonable periods of time and of achievingat least satisfactory separations, commercial acceptance thereof hasbeen lacking, due at least in part to the fact that very large,expensive mechanical devices were required, construction of which had tomeet the highest standards to avoid the everpresent hazards present whendealing with large volumes of molten materials moving at high speed. Thehazards are particularly great, of course, when molten alloys are beinghandled. Additionally, the necessary high degree of temperature andheat-flow control required to successfully carry out the process isdifficult to achieve when working with such cumbersome equipment.

It is accordingly an object of the present invention to provide aprocess wherein substantial volumes of material maybe purified within areasonable time.

It is a further object of the invention to providea process forfreeze-refining materials which can be carried out on a routine,production basis with a simple equipment.

It is yet another object of the invention to provide an improvedfreeze-refining process wherein heat-extraction may be readilycontrolled with a high degree of accuracy.

Various other objects and advantages of the invention will appear fromthe following description of several embodiments of the invention, andthe novel features will be particularly pointed out hereinafter inconnection with the appended claims.

The present invention, briefly stated, improves upon previous dynamicfreezing processes by employing stationary mechanical elements only, andletting the liquid provide its own agitation by the action of gravity.Thus, in its simplest embodiment, the invention comprises a containingvessel holding a quantity of material which is to be refined, the vesselbeing located above the refining apparatus and providing a static headof pressure for the operation thereof. A conduit consisting of eithertube, pipe or covered launder connects with the bottom of the containingvessel. This conduit passes through a zone of controlled temperature or,when desired, temperature gradient, and as molten material passestherethrough in gravity-flow, the higher melting fraction freezes to thewalls thereof, and the lower melting fraction passes through as aliquid. The length, diameter and slope of conduit employed, which mayconveniently be wound in the shape of a spiral or helical coil toconserve space,

is calculated to be sufficient to accommodate the capacity.

of the containing vessel without becoming clogged or otherwise impedingthe flow of molten material. After a charge of material has passed oneor more times therethrough the lower-melting liquid removed, thetemperature in the controlled zone around the tube is increased and thehigher-melting fraction allowed to flow out.

As is well known, the flow of a liquid through a tube can be eitherlaminar or turbulent. While deposition of the higher-melting fractionfrom a stream flowing in a turbulent manner is found to be moresatisfactory than from a laminar stream, as might be expected, it hasbeen found to be entirely possible to achieve satisfactory depositionfrom a laminar stream under most conditions. The conditions of freezingare found to be comparable to those in a rotating drum, i.e., the flowof molten material is substantially parallel to the deposition surface,so it is not surprising that deposition takes place under both sets ofconditions.

While not wishing to be bound by any particular theory of operation, itis believed that the theories controlling the operation of zone refiningare equally applicable to the invention. Of course, it must beremembered that the values assigned to various operating parameters willdiffer by several orders of magnitude. The validity of this cor- 3relation is illustrated, for example, by examination of a thincross-section of the tube of the invention. At a given instant of time,this cross section will have a solid outer section (the tube itself), a'solid ring of frozen material located therein, .and the middle willcomprise a molten core. Thus, instead of a molten zone advancing througha solid rod, the molten zone is effectively Within the rod, the walls ofwhich gradually close in about it. It can thus be theorized that thesame general boundary conditions which apply to zone refining may alsobe applied to the invention. In this regard, it is important torecognize that three separate boundary conditions exist; a momentumboundary layer, a diffusion boundary layer, and a thermal (energy)boundary layer. While the complete set of boundary layer equations whichdescribe the combined momentum, mass, and energy transport have beenworked out for a two-dimensional, steady laminar boundary layer byEckert and Drake (Heat and Mass Transfer, McGraw-Hill Book C0,, NewYork, p. 456 (1959)), they are most difficult to solve. By makingcertain assumptions, these equations can be substantially simplified, asshown by Johnston and Tiller (Fluid Flow Control During Solidification:Magnetic Stirring in the Plane of the Solid-Liquid Interface, AIMETransactions, 1961, vol. 221, p. 331), and the operability of theinvention can be confirmed on a theoretical level as well as thepractical. The latter is attested by the examples append-ed hereinbelow.

It is believed that a better understanding of the invention will begained by referring to the following detailed description thereof, takenin conjunction with the single drawing, which is a simplified schematicview of an embodiment of the apparatus utilized in carrying out theinvention with high-melting materials, such as alloys.

With reference to the drawing, it will be seen that a conventionalrefractory crucible 1 is employed to contain the molten alloy 2. Thecrucible 1 is fitted with a bottompouring device 3 of the type wellknown in the art. Of course, any convenient means may be employed tocontain the raw material; when low-melting materials are being treated,such as, for example, organic waxes, no special precautions need betaken to keep the material molten. With alloys and other high-meltingmaterials, however, it is often convenient to employ heating means suchas the heating coil 4 to maintain the melt at a uniform temperature.

Hot metal 2 from the crucible 1 is poured into the holding tank 5, whereit provides a suitable head of pressure for the apparatus. From holdingtank 5, the metal flows by gravity into and, in part, through the tubing7, illustrated as having been formed in a helical coil.

The coiled tubing 7 is maintained under conditions suitable for carryingout the freeze refining within a heat and temperature controlled area,indicated gene-rally by the dotted lines 6. The nature of the heat andtemperature control required will, of course, vary with the materialbeing treated, the heat transfer characteristics of the tubing 7 and thetemperature and other properties of the raw material. Thus, for example,an agitated ice-water mixture is satisfactory for the freeze-refining ofwater-peroxide systems. With many organics, a tank of water at asuitable temperature is operable. For certain low-melting alloys aheated oil bath is preferred, and for other alloys, where substantiallyelevated temperatures are required to keep the lower-melting fractionmolten throughout, various furnace type structures and a gaseous mediumare required.

In order to obtain proper freeze-refining it is necessary, of course, tomaintain both temperature and rate of heat exchange within thecontrolled area 6 as constant as possible. While in some instances it ispossible to maintain the required conditions over a sufficient period oftime by merely providing a large tank of cooling medium, as, forexample, the aforementioned ice-water mixture, it is generally necessaryto provide heat transfer means 9 and a suitable pump 10 for circulationof the cooling medium.

By providing a heat exchange 9 with a capacity sufficient to extractheat from the cooling medium at substantially the same rate that theheat of solidification of the freezing liquid is evolved from the tube7, it is possible to maintain conditions substantially constant withinthe controlled area 6.

Where liquid cooling mediums are employed, it is necessary that vigorousagitation be employed, so that temperatures throughout the controlledarea are substantially constant; where gaseous mediums are used thenatural cirsulation due to action of the pump 10, coupled with properdesign of the enclosure, is generally sufficient.

The materials of construction are dependent, again, primarily on theparticular temperatures required to separate the mixture being treated.Those skilled in the art will fully realize what materials are bestsuited for a given system; use 'of several materials is illustrated inthe appended examples.

While the optimum conditions of operation of the invention for anyparticular raw material may be calculated from empirical data, thecalculation is sufiiciently complex to be impractical for mostoperations. Moreover, such calculations will only give a reasonableapproximation of operating conditions, as in many cases the empiricaldata available is inexact, and in most cases the applicable equationshave been worked out for ideal systems in a laboratory rather than on aproduction scale. Also, many assumptions have been made in theirdetermination. Thus, one skilled in the art, with a phase diagram of thesystem to be treated and knowledge of the heat capacities of thecomponents of his equipment and heat of solidification of thehigher-meltin g fraction, will be able to estimate the conditionsrequired for a proper separation of components, and with the aid of afew trial-and-error runs will be able to determine the optimumconditions for the particular system involved.

It is worthy of emphasis to note that, in the course of any given run ofthe apparatus, the optimum conditions Will be subject to some change.While in most instances this will not be of sufiicient magnitude torequire compensation of the heat exchange, there are occasions when thismust be done. Thus,- as freezing along the walls of the tube proceeds,the heat capacity of the wall builds up, and the volume of moltenmaterial passing through the tube (per unit time at constant pressure)will decrease. Thus, in certain instances it is necessary to reduce therate of heat exchange or build up fluid pressure in the tube. Thisproblem is easily remedied, however, by use of interlocking controlsbetween temperature-sensing means located outside the tube 7 and theheat exchanger pump.

It is important to remember that any alloy freeze-refining operation is.limited, in terms of refining efliciency, to the phase relationshipsexisting between the components of the system being treated. Thus, forexample, where a eutectic with or without peritectics exists in thephase diagram and the raw material contains a greater amount of thesolute element than is required for the eutectic alloy, thelower-melting fraction will approach the eutectic composition butfurther purification cannot be achieved. Of course, the higher-meltingfraction may be treated in successive stages until a suitable purity isattained. In all cases the lever rule will hold true and determine thecomposition of the lowerand hi-gher-melting fractions, and a program ofmultiple stage refining will be readily worked out by one skilled in theart. A convenient method is to employ several conduits or tubes runningfrom one feeding vessel. The feed liquid can then be fed to each tubefor a certain period of time in seriatim (i.e., one after the other),and the higher-melting fraction removed from the tube by re-heatingimmediately after it stops receiving the liquid. In this manner theprocess is rendered substantially continuous, and the two fractionsrecovered can be conveyed to other units which will achieve furtherpurification or recycled to the first unit for individual treatment.

The removal of the collected solid from the tube is a relatively simplematter, merely requiring that the temperature within the heat-controlledarea 6 be raised above the melting point of the solid higher-meltingfraction. When the melting point is below roomtemperature, this may bedone by draining the enclosure; at higher temperatures it can be done byadjusting the temperature of the medium employed. I

'In the following examples, it will be convenient to discuss theefiiciency of separation in terms of a separation factor or K factor. Aconvenient form of K factor which has been used by other workers in thefield is as follows:

where S is the amount of impurity or solute (i.e., m-ass),

remaining in the liquid after a fraction g has been frozen,

and S is the amount of impurity or solute contained in the originalmelt. The factor K can be defined as a measure of the weight fraction ofthe solute element (S/S concentrating in the liquid when a given volumefraction (g) has solidified. It is to be noted that this expressiondiffers from the K factor which is obtained by dividing the percentimpurity in the liquid into the percent impurity of the Example I Froman aluminum alloy smelter 412 pounds of a 70%-Al30% W alloy are producedin a 30-minute period. This alloy is freeze refined into a 98.6% Alfraction by passing it through a series of alumina launders arranged inthe shape of a helix so that one launder discharges directly into thenext. The alloy is held in a heated bottom pour vessel where it remainsabove 1250 C. by virtue of the superheat from the smelting furnace andthe heat in the vessel. The bottom pour plunger is adjusted so that the412 pounds of alloy is metered into the launder over the half-hourperiod.

The launders are fired alumina troughs, 1 /2 inches inside diameter and3 /2 feet long, with 4 inch walls. Each launder is covered with a A"alumina plate and the launder is supported in the middle by refractorybrick. The helix formed by the launders has a 5 foot diameter with fourlanuders to a loop. There are ten loops, each with a slope. A pump (notshown) at the discharge of the helix recycles the aluminum alloy to thetop launder but not to the holding vessel.

The alumina helix is placed in a refractory brick enclosure and held bymeans of a closely controlled combustion process to 705 C.-* -1 C. Thealuminum-tungsten alloy is then recycled through the helix launder for30 minutes, after which time 203 pounds of liquid remains in the helixstructure. This liquid is taken from the bottom of the helix andanalyzes 98.6% Al. The gases in the brick enclosure are then heated tobring the helix above 1300 C.; this causes the alloy solidified on thewalls of the launder to melt and discharge at the bottom of the helix.Two hundred eight pounds of the high melting alloy is collected. Thisalloy analyzes 42% Al and is recycled to the smelting furnace.

The temperature of the brick enclosure is then adjusted to 666 C.il C.,and the 98.6% Al liquid is then circulated for further purification.After one hour the liquid has reached a composition of 99.2% Aland nofurther change occurs at that temperature. Two hundred pounds of thepurified liquid are obtained.

6 The heating chamber is then heated to 750 C. and the deposited solidmelts and is discharged. This material amounted to 3 pounds and analyzed64% Al.

Example II For this test, a laboratory unit consisting of a holdingvessel maintained at a temperature of 5 C. is employed. The fluid levelin the vessel is maintained constant at a 6 inch level by pumping in onegallon per minute of a 10% NaCl aqueous solution. The holding vessel isconnected with any one of eight identical cooling coils. The coolingcoils consist of 30 feet of inch I.D. copper tubing wound into a helicalspiral. The diameter of the spiral is 16 inches and each turn is spacedA inch apart. During the cooling cycle the coil is maintained at l9 Ci1C., and during the melting cycle it is allowed to come to roomtemperature.

The solution is allowed to flow under its own head through the firstcoil until pressure in the line increases, as shown by an increase inthe height of the solution in the holding vessel. A concentrated brinesolution containing 24.8% NaCl is discharged from the end of the helix.The pressure build-up in the line is the result of freezing of purifiedwater, and usually occurs'after 15 to 20 minutes of operation. At thistime fiow is diverted to the next cooling coil and the refrigerantremoved from around the first coil. This coil is allowed to come to roomtemperature and after one hour a quantity of pure water is discharged.

The operation was continued until the 8th coil had been used, at whichtime the stream was again diverted back to the first coil. Thus, it waspossible to operate the unit continuously. The average production fromthe unit was 36 gallons of purified water and 24 gallons of concentratedbrine per hour.

Example III The same experimental equipment and conditions employed inExample II were used to separate a dilute solution of hydrogen peroxideand water. In a 1-hour run, a very satisfactory K factor of 0.13 wasobtained. It is to be noted that the apparatus and process of theinvention Will provide even better separations with concentrated HOOHsolutions (i.e., where conventional distillation procedures involvesubstantial hazards.

It will be understood that various changes in the details, materials,steps and arrangements of parts, which have been described andillustrated in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims.

Having thus described my invention, what it is desired to secure byLetters Patent is:

1. Process for freeze-refining liquids having at least two componentswith different melting points that comprises, maintaining said liquid ata pre-selected temperature and at a pre-selected static pressure;feeding said liquid by gravity flow into an elongated, inclined,stationary conduit, said liquid flowing as a laminar stream; maintainingsaid conduit at a temperature between the melting points of thehigher-melting and lower-melt-' ing of said components, whereby ahigher-melting fraction is frozen onto the walls of said conduit;

recovering a lower-melting fraction in liquid form at the lower end ofsaid conduit;

discontinuing said gravity fiow of liquid;

removing said lower-melting fraction;

raising the temperature of said conduit above the melting point of saidhigher-melting fraction frozen therein and melting the same; and

recovering said higher-melting fraction in liquid form at the lower endof said conduit.

3,254,989 '7 3 2. The process as claimed in claim 1, wherein a plu-References Cited by the Examiner rality of conduits are employed andeach is fed with said UNITED STATES PATENTS liquid in seriatim for apre-selected length of time, after which feeding the temperature israised so as to melt Re'23958 3/1955 62123 out the solid frozen therein,and whereby said process is 5 2,471,899 5/1949 f rendered substantiallycontinuous. 2801162 7/1957 Keeping 75 63 3, The process as claimed inclaim 1, wherein one of 2969970 1/1961 S'chomer 7593 said fractions isre-treated according to claim 1 until a D AVID L RECK Primary Examine].substantially pure fraction is obtained. t

4. The process as claimed in claim 3, wherein the 10 BENJAMINHENKINExami'wr' other of said fractions is also re-treated. C. N.LOVELL, Assistant Examiner.

1. PROCESS FOR FREEZE-REFINING LIQUIDS HAVING AT LEAST TWO COMPONENTSWITH DIFFERENT MELTING POINTS THAT COMPRISES, MAINTAINING SAID LIQUID ATA PRE-SELECTED TEMPERATURE AND AT A PRE-SELECTED STATIC PRESSURE;FEEDING SAID LIQUID BY GRAVITY FLOW INTO AN ELONGATED, INCLINED,STATIONARY CONDUIT, SAID LIQUID FLOWING AS A LAMINAR STREAM; MAINTAININGSAID CONDUIT AT A TEMPERATURE BETWEEN THE MELTING POINTS OF THEHIGHER-MELTING AND LOWER-MELTING OF SAID COMPONENTS, WHEREBY AHIGHER-MELTING FRACTION IS FROZEN ONTO THE WALLS OF SAID CONDUIT;RECOVERING A LOWER-MELTING FRACTION IN LIQUID FORM AT THE LOWER END OFSAID CONDUIT; DISCONTINUING SAID GRAVITY FLOW OF LIQUID; REMOVING SAIDLOWER-MELTING FRACTION; RAISING THE TEMPERATURE OF SAID CONDUIT ABOVETHE MELTING POINT OF SAID HIGHER-MELTING FRACTION FROZEN THEREIN ANDMELTING THE SAME; AND RECOVERING SAID HIGHER-MELTING FRACTION IN LIQUIDFORM AT THE LOWER END OF SAID CONDUIT.